<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <h1>我是a页面</h1>
    <button class="but">关闭页面</button>

    <script>
        var but = document.getElementsByClassName('but')[0]
        but.onclick = function() {
            window.close()
        }
    </script>

</body>

</html>